developing a physical bim library for building thermal energy simulation
TRANSCRIPT
Automation in Construction 50 (2015) 16–28
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Automation in Construction
j ourna l homepage: www.e lsev ie r .com/ locate /autcon
Developing a physical BIM library for building thermal energy simulation
Jong Bum Kim, WoonSeong Jeong, Mark J. Clayton, Jeff S. Haberl, Wei Yan ⁎Texas A&M University, United States
⁎ Corresponding author at: 3137 TAMU, College StationE-mail addresses: [email protected] (J.
(W. Jeong), [email protected] (M.J. Clayton), [email protected] (W. Yan).
http://dx.doi.org/10.1016/j.autcon.2014.10.0110926-5805/© 2014 Elsevier B.V. All rights reserved.
a b s t r a c t
a r t i c l e i n f oArticle history:Received 30 December 2013Received in revised form 4 August 2014Accepted 21 October 2014Available online xxxx
Keywords:Building Information Modeling (BIM)Building energy modeling (BEM)Object-Oriented Physical Modeling (OOPM)Interoperability
Insufficient interoperability resulting from complex data exchange between architectural design and building en-ergy simulation prevents the efficient use of energy performance analyses in the early design stage. This paperpresents the development of a Modelica library for Building Information Modeling (BIM)-based building energysimulation (ModelicaBIM library) using an Object-Oriented Physical Modeling (OOPM) approach and Modelica,an equation-based OOPM language. By using the ModelicaBIM library, our project investigates system interfacesbetween BIM and energy simulation, which can perform semi-automatic translation from the buildingmodels inBIM to building energymodeling (BEM) using a BIM's authoring tool's Application Programming Interface (API).TheModelicaBIM library consists of OOPM-based BIM classes and OOPM-based BIM structure. OOPM-based BIMclasses represent building component information. OOPM-based BIM structure consists of test case models thatdemonstrate (i) howbuilding information in BIM can be transformed to OOPM and (ii) how design operations inBIM, such as changing a building geometry and editing building components, can be translated into BEM. A casestudy for simulation result comparisons has been conducted using (i) OOPM-based BIM models in theModelicaBIM library and (ii) LBNL Modelica Buildings library (a Modelica-based building thermal simulation li-brary developed by Lawrence Berkeley National Laboratory). Our implementation shows that the ModelicaBIMlibrary enables (i) objects in BIM to be translated into the OOPM-based energy models and (ii) existing OOPMlibrary to be utilized as a simulation solver for BIM-based energy simulation.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
Since 1996,more than four hundred software tools for building energysimulation have been listed in the “Building Energy Software Tools Direc-tory”providedbyU.S. DOE (2013). Among the tools, a fewaredominantlyused in education and industry [1–5]. For example, DesignBuilder, DOE-2,eQuest, Ecotect, Energy-10, EnergyPlus, Green Building Studio, HEED, andIESVE are widely used in the United States [6]. In education, Ecotect,Energy-10, Radiance, CONTAM, and eQUEST are often used [3].
Recently, linking Building Information Modeling (BIM) and energysimulation either with standard data schemas such as the IndustryFoundation Classes (IFC) or with common data formats such as GreenBuilding XML (gbXML) is a developing area in research. Some toolswere modified to utilize BIM and others were developed to be compat-ible with BIM authoring tools [5]. In this section we trace approachesand limitations of current BIM-based energy simulation.
1.1. Generating energy simulation models from BIM
Most energy simulation tools consist of the simulation engine and thegraphical user interfaces (GUIs). The engines were often written in
, TX 77843, United States.B. Kim), [email protected]@tamu.edu (J.S. Haberl),
imperative computer language such as FORTRAN, C, and C++ [7]. TheGUIs facilitate rapid input and output processing as well as simulationrunning when the semantics of input files, parameters of simulation set-tings, and the formats of output files are dependent on the simulation en-gines. In many cases, simulation engines and GUIs were developedseparately. For instance, Hevacomp and DesignBuilder are independentlydeveloped GUIs of EnergyPlus.
Some existing simulation tools and their GUIs were modified fordata exchange capabilities between BIM and energy simulation throughstandard data schemas such as IFC and gbXML, which contain buildinggeometry information and other information of internal loads, occupan-cy, zone assignments, system configuration, and utilization schedules[4]. Currently, both schemas are supported by BIM tools includingRevit, Bentley, and ArchiCAD, as well as energy simulation tools suchas Green Building Studio, Ecotect, Hevacomp, eQUEST, HAP, andIES b VEN [8].
RIUSKA, a GUI of the DOE-2.1E engine, has an IFC import feature thatcan obtain building geometry information fromBIM. eQUEST supports di-rect imports of DWG and gbXML with limitations: only two-dimensionalbuilding footprints can be obtained fromDWG [9]; and some eQUEST fea-tures are not usable for the gbXML model [9].
Many applications and GUIs are developed for EnergyPlus such asSimergy, DesignBuilder, OpenStudio, CYPE-Building Services, DemandResponse Quick Assessment Tool, Easy EnergyPlus, EFEN, AECOsim,Hevacomp, and SMART ENERGY [10]. OpenStudio is a collection of appli-cations: a SketchUp plug-in as a building geometry editor; anOpenStudio
17J.B. Kim et al. / Automation in Construction 50 (2015) 16–28
application as amain energymodeling interface; RunManager as a simu-lation interface; and the ResultsViewer [11]. OpenStudio can importgbXML in company with material, construction, surface, space, building,and schedule information from BIM. DesignBuilder can import three di-mensional geometry information through gbXML and two dimensionalbuilding footprints through DXF. OpenGL solid modeler of DesignBuildervisualizes building façade design and solar studies [10]. Simergy, a GUI ofEnergyPlus, supports editing building geometries and HVAC systems,generating EnergyPlus input (IDF) files, running EnergyPlus simulations,and reporting simulation results [12]. Building geometry information inCAD/BIM models can be imported into Simergy through IFC and gbXMLor can be generated using the Simergy GUI. Simergy is developed basedon Simulation Domain Model (SimModel) that is a XML-based datamodel [13]. Hevacomp, another GUI of EnergyPlus, can load building ge-ometry and construction information through gbXML, DWG, and DXF,and then generate the EnergyPlus input files.
1.2. Integrating energy simulation tools with BIM authoring tools
Some simulation tools can be plugged into BIM authoring tools asadd-ins such as IESVE and Green Building Studio, which allow energysimulation within the BIM environment. Green Building Studio is aweb-based energy analysis environment based on DOE-2.2. Energymodels are generated from Autodesk Revit models via certain manualpreparation processes. Then, simulation is performed by a cloud serviceand the results are reported to users. IESVE has a Revit plug-in that cangenerate an energy model based on gbXML in Revit, but editing theexported energy model can be done in the IESVE interface.
Ecotect performs various analyses for solar, daylighting, acoustic,thermal, and airflow using multiple engines such as Radiance, DAYSIM,EnergyPlus, and Ecotect built-in engines. Ecotect can import gbXML,IFC, DXF, etc. fromBIM [14]. In addition, Vasari [15], a parametric concep-tual building design tool, can generate a Green Building Studio thermalmodel and perform solar studies, solar radiation analysis, andwind stud-ies. In sum, these tools and their GUIs can read partial information of abuilding model from BIM through IFC or gbXML.
1.3. Limitations
While energy modeling using data schemas such as IFC and gbXMLhas been implemented for many simulation programs, reliable energymodels can be acquired throughmanualmodel checks andmodification[16]. Although energy simulation tools have been modified or devel-oped to support the energymodel generation process using BIM, the en-ergy simulation analysis in design is still regarded as cost and laborintensive [17].
Generating an energy simulation model from BIM is time consum-ing, error-prone, and not intuitive [18]. Incongruent information storedin BIM and BEM requires a certain level of translation from BIM to BEM.First, all model information of BIM does not need to be translated intoBEM. For instance, a room in an architectural model does not always in-dicate a zone in an energy simulation model. Second, some values re-quired for energy modeling are not imbedded in BIM. For example,neither thermal zone information nor boundary conditions are storedin BIM. Often, model information in BIM is abstracted in BEM. A wall, aroof, and a floor in BIM are simplified as surfaces in energy simulation.When energymodeling is conducted for a buildingwith complex geom-etry, manual model inspections become more complex [19]. Studies onBIM-based energy modeling describe that standards or guidelines arerequired to relieve the problems in building geometry translationfrom BIM to BEM [20,21]. On the other hand, incorporating simulationresults into the design stage is not always obvious [17]. For designers,what the simulation results imply and how building design can corre-spond to such results are not clear. For instance, whole building levelsimulation results such as annual energy consumption and peak loadsare often provided in a format of charts, spreadsheets, and plots. They
do not indicate which and how building components need to be modi-fied to achieve a specific energy performance.
Although the functionalities of the building energy simulationtools have been developed, further work is expected to improvethe interoperability between BIM and the energy simulation tools[22]. For BIM-based energy simulation in early design, softwaretools need to have intuitive GUIs [2], seamless data exchange capa-bilities [19], reliable BIM compatibilities [5], and automated rule-based translation capabilities from BIM to BEM [18]. In addition,tools' good maintainability that allows modification and customiza-tion of calculation modules may improve the use of building energysimulation [23].
2. ModelicaBIM library, Revit2Modelica framework, and PBIMresearch
Our research investigates a new interface for BIM-based building en-ergy simulation, integrating the architectural design and the energysimulation process. The objectives are: (i) enhancing the interoperabil-ity among BIM and BEM, (ii) enabling more reliable BEM generationfrom BIM, (iii) enhancing the integration and the coordination amongmulti-domain BEM, and (iv) enabling BIM as a common user interfacefor the multi-domain energy simulations. To do so, we investigated aBIM-based energy modeling and simulation framework, which inte-grates BIM andmulti-domain energy simulations, and named it as Phys-ical BIM or PBIM [24]. Building thermal, daylighting, and BIPV analysesare being implemented in the PBIM framework, and this paper describesour research on the BIM to thermal BEM in detail.
Revit2Modelica is a framework of BIM-based building thermal simu-lation (Fig. 1). Revit2Modelica consists of Revit Application Program-ming Interface (API) programs written in C# programming languageand the Object-Oriented Physical Modeling (OOPM)-based library(ModelicaBIM library) written in Modelica. Revit API programs havefunctions that add data sets to BIM, access the BIM data structure, andproduce Modelica models using the ModelicaBIM library and LawrenceBerkeley National Laboratory (LBNL) Modelica Buildings library. TheModelicaBIM library consists of a ModelicaBIM Class package and aModelicaBIM Structure Example package. The ModelicaBIM Class pack-age contains building element classes that use the classes in the LBNLModelica Buildings library. The need of wrapping classes is driven bythe different object semantics and structures between BIM and BEM.Detailed wrapping processes applied in the ModelicaBIM library devel-opment will be explained later.
A workflow of Revit2Modelica is illustrated in Fig. 2. Revit2Modelicatranslates BIM into OOPM in Modelica, semi-automatically generatesBEM (Modelica models) from BIM (Revit models), immediately per-forms building thermal analyses, and reports simulation results.
• First, the prototype accessesmodels in BIM to readmodel informationsuch as geometry, materials, and location information.
• Second, the prototype transforms the obtained information by follow-ing the model structure of the ModelicaBIM library.
• Third, by using the transformed model information, the prototypeoutputs aModelica BEMand calls Dymola, an IntegratedDevelopmentEnvironment (IDE) for Modelica, to run simulation.
• Fourth, the prototype reports simulation results in the BIM user in-terface. The prototype currently analyzes heat flow of each build-ing component, free floating indoor temperatures, accumulatedheating and cooling loads, as well as peak heating and coolingloads. Plots of the simulation results are generated and opened inthe Revit user interface.
Three key milestones in the Revit2Modelica development are:
• developing an OOPM library (ModelicaBIM library) including OOPM-based classes andOOPM-basedmodels to link BIMand existingOOPMsimulation solvers,
PBIM Research
Integrated Simulator and Performance Analyzer
Revit2Modelica(Solar Thermal)
Revit2Radiance(Daylighting)
Revit4BiPV(BiPV)
ModelicaBIM Applications (Revit API)
ModelicaBIM Library
LBNL Modelica Buildings Library
Radiance / DaySIM
Radiance BIM Applications(Revit API)
BiPV BIM Applications (Revit API)
Fig. 1. PBIM research framework. The ModelicaBIM library is a part of BIM-based solar thermal research.
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• prototyping the system interfaces using BIM API, and• validating the interfaces and the library through simulation resultcomparisons.
This paper describes the development of theModelicaBIM library. Thefollowing sections explain the framework of the library development,goals and challenges, methods and tools, descriptions of the library com-ponents, as well as validation of test case models in the library.
3.ModelicaBIM library development for BIM-based energy simulation
This section describes tools and data, challenges and objectives, aswell as methodologies in the library development.
3.1. Tools and data
3.1.1. BIM and the BIM authoring toolsBIM aims to support information creation, exchange, and applica-
tions in the buildings' lifecycle in the Architectural Engineering Con-struction (AEC) industry. BIM ties all the building components with
Fig. 2. A workflow of
imbedded information to create a building product model [25,26]. BIMstores both geometric descriptions and non-geometric attributes in itsobjects and parameters. Semantically rich information can be storedinto the shared database. Parameterization capabilities in BIM enablethe rapid generation of complex forms and interactive model changes.
BIM authoring tools provide API that enables software developers toextend the tools, e.g. Autodesk Revit's API in C# and VB.NET; GraphisoftArchiCAD's API in C and C++; BentleyMicroStation'smacros in VB.NET,C++, and C#, as well as MicroStation Development Libraries in C. It al-lows complex geometric modeling, accessing information of BIM ob-jects, comprehensive database creation, and presentation of analysisoutput associated with corresponding BIM objects. In our project, in-stead of using IFC or gbXML, we propose a BIM API method to accessthe BIM data directly from the BIM authoring tool in order to take ad-vantages of the parametric modeling capability of BIM and a moreseamless integration with less data conversions between BIM and BEM.
3.1.2. OOPM and ModelicaOOPM is a fast-growingmodeling and simulation approach, offering
a structured and equation-based modeling [27]. Modelica is an OOPM
Revit2Modelica.
Table 1An example of roof object mapping.
Category BIM ModelicaBIM
Object Roof PBIM.BIMPackage.RoofOrientation Origin/normal (vector) Tilt/AzimuthArea Area AreaThickness Width ThicknessMaterial DefaultRoof PBIM.BIMPackage.MaterialThermal resistance Resistance (ft2·°F·hr/BTU) R (m2·K/W)Number of layers CompoundStructure.LayerCount nLayOther physicalproperties
Roughness RoughnessN/A AbsorptanceN/A Emittance
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language that models the dynamic behaviors of technical systems usingdifferential algebraic equations (DAE)-based simulation [27]. It has beenused in the complex physical system design of mechanic, electric, ther-mal, and control systems. A component connection diagram inModelicacan present physical system topology of energymodels [27]. The object-oriented modeling approach of Modelica may enable an intuitive map-ping from the object-based BIM structure to OOPM.
The use of Modelica needs aModelica library offeringmodel compo-nents and solvers as well as a Modelica simulation environment. In thisresearch,we useDymola [28] as a simulation environment and the LBNLModelica Buildings library as a simulation solver.
3.1.3. LBNL Modelica Buildings libraryThe LBNL Modelica Buildings library contains dynamic models and
control systems for building energy simulations [23]. It supports thesimulation of heating and cooling systems, controls, heat transferthrough building envelope, as well as airflow models. The library isbeing developed and some packages and components have been vali-dated. TheHeatTransfer and Roomspackages in the library aremajor re-sources for the building thermal analysis, and they have been tested andvalidated through benchmark simulationmodels [29,30]. The executiontime test of Modelica simulation shows that it can be comparable withTRNSYS [31].
To construct a building energymodel with the LBNLModelica Build-ings library, a user needs to manually retrieve a building's data andwrite Modelica codes that represent a building's energy model. Oncethe BIMmodel is preprocessedmanually, our system automatically con-verts models in BIM into Modelica-based energy models by using theModelicaBIM library and then performs thermal analysis using theLBNL Modelica Buildings library. This automated transformation fromBIM to BEMwill speed up the design and the simulation process signif-icantly and substitute operators' subjective interpretations to minimizehuman errors in energy simulation.
3.2. Challenges and objectives
3.2.1. Apply a coherent object relationship from BIM to BEMIn the energy modeling process, object relationships and semantics
of architectural models are often abstracted when suchmodel informa-tion does not substantially affect simulation results. For example, build-ing components such aswalls, roofs, and floors are frequently simplifiedas surfaces and then heat transfer through the surfaces are simulated inthe energy models. When designers modify building design accordingto the abstracted simulation results, energy performance of each surfaceneeds to bemapped to corresponding building components again. Suchabstraction can delay the result interpretation in building design.
Consistent object classifications and semantics between BIM and BEMare applied in theModelicaBIM library development. OurModelica-basedsimulationwill performcomponent-level analyses and the simulatedper-formancewill be forwarded to Revit to informdesign. Applying consistentobject classifications of building components from BIM to BEMwould besignificant to pair simulation results with BIM.
3.2.2. Create parameters to receive values from BIMIn the LBNL Modelica Buildings library, predefined materials such as
brick, concrete, and insulation boards are used. To add new materialsthat are not defined in the library, users need to create newmaterial ob-jects prior to the use. The use of manually-predefined material classesmakes both modification and creation of materials time consuming. InRevit2Modelica, we transform material properties stored in BIM intothe parameters of the Modelica energy models. To do so, classes in thelibrary are created to have parameters that can store properties in BIM.
3.2.3. Create objects for diverse building geometryNew objects are created to have parameters flexible for diverse
building geometry. For example, width and height of a surface are
used to calculate the surface area in the LBNLModelica Buildings library,which is limited to rectangular surfaces. We use the area value insteadof width and height to represent diverse geometry and the values areobtained from BIM directly.
3.3. Methodology
Our methodologies in the ModelicaBIM library development are(1) object mapping for semantics and behaviors, (2) preprocessingBIM, and (3) wrapping existing solver classes with new library classes.
3.3.1. Object mapping for semantics and behaviorsObject mapping allows us to identify required information and a
structure of the ModelicaBIM library. Mostly, we addressed objectsemantics and behavior mismatches that can impede an automaticmodel transformation from Revit to Modelica. The two mismatcheswe defined in this paper are (1) semantic mismatches of buildingcomponents in BIM and BEM and (2) behavior mismatches betweenarchitectural design and energy modeling. While we implementedan object mapping between Revit and Modelica, we describe se-mantic mismatches using generic concepts that are not software-dependent.
Semantic mismatches of building components can delay data ex-change of building objects and their attributes between two domains.For instance, a building envelope in BIM is commonly decomposedinto walls, roofs, and floors/slabs, while it can be recognized as exterioror interior surfaces in BEM. An interior space is recognized as a room inBIM, while it can be represented with a thermal zone in BEM. Precisegeometry information of building components is stored in BIM, but awetted surface area is required for one-dimensional heat transfer calcu-lation in BEM.
Object mapping for the roof object is exemplified in Table 1. Tomap the roof object, some values can be directly used, but othersneed to be either translated or created. For instance, the originand normal vectors are used in BIM, while the tilt and azimuthvalues are needed in BEM for the coordinate system; therefore,the coordinate system is transformed. When different units areused such as thermal resistance, unit conversion is carried out.Some missing values in BIM such as absorptance and emittanceare added in BEM.
Behavior mismatches during the modeling process can defer inter-active energy model update in accordance with building design chang-es. For example, adding a door on the exterior wall can be interpreted asadding an opening that causes air flow and air infiltration. Separatingone room with an interior wall in BIM can be translated into BEM asadding an interior surface that performs heat transfer calculation be-tween two thermal zones.
The object mapping method establishes a set of rules for an auto-matic data transformation, which will be applied to the Revit2Modelicaprototype development. This object mapping method enables the
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Revit2Modelica prototype to (1) transform each BIM object (that is ther-mally significant to building energy consumption) into aModelica ener-gy object by using the ModelicaBIM library classes; (2) generateModelicaBIM models by applying the ModelicaBIM Structure; and(3) allow the ModelicaBIM to call the publically available LBNLModelica Buildings library as a simulation engine. Below we describethe detailed development process of the ModelicaBIM library and theModelicaBIM Structure.
The significance of comprehensive information exchange betweenstakeholders has been investigated in research on the Model View Def-inition (MVD). MVDs have been widely accepted in information ex-change using IFC, so they are often understood as subsets of the IFCmodel specification [32]. For instance, a set of MVDs are proposed in“Concept Design BIM 2010”, enabling four types of analyses from an ar-chitectural model: spatial program validation, circulation and securityanalysis, energy performance analysis, as well as quantity takeoff andcost estimating [33]. Amore generic definition ofMVDs is a comprehen-sive representation of the information concepts required for particularinformation exchange among multiple domains [34]. Therefore, thegeneral concept can be captured when our library is created for multi-domain energy simulations.
3.3.2. Preprocessing BIMWe preprocessed BIM by using the modeling capabilities of the BIM
authoring tools in this paper. The automated preprocessing of BIM willbe implemented in the future research. Some of the numerical values inBIM are directly usable, but others need to be transformed according tothe appropriate data transformation rules between BIM and Modelica.In our development, BIM is preprocessed by addition, translation, andcalculation: addition is to add data sets that are absent in BIM but need-ed for physical modeling such as solar and infrared absorptance ofbuilding materials; translation is to translate data between BIM andBEM such as the room-to-thermal zone translation; and calculation isto populate new values from included values in BIM such as awindow-frame ratio.
Some simple preprocesses in BIM enable building geometry abstrac-tion. For example, a building in Fig. 3 consists of four exterior walls, oneinterior wall, one floor, and a roof. The colored wall attached to tworooms is one piece in BIM, but it needs to be recognized as two piecesin energy modeling. A required preprocess in BIM is splitting the wallinto two pieces and the same preprocesses can be applied to the roofand the floor. While the preprocessing in this paper is performed man-ually in BIM, they will be substituted with automatic preprocessing inour future research based on a rule based conversion for building topol-ogies and space boundaries [20,21,35].
3.3.3. Wrapping solver classes with the ModelicaBIM libraryA class wrapping method enables an intermediate Modelica energy
model not only to follow data structures and semantics of BIM butalso to use existing Modelica libraries as a simulation engine. It facili-tates objectmapping fromBIM toModelica energymodels. Objectmap-ping from BIM to the LBNL Modelica Buildings library is accomplishedby the wrapper classes created in the ModelicaBIM Class package.The ModelicaBIM Class package contains wrapper classes of the LBNL
Fig. 3. Preprocessing in BIM of a building envelope.
Modelica Buildings library. Primary building components for buildingdesign have been implemented. For instance, the Room class of theModelicaBIM library is a wrapper class of the MixedAir class of theLBNL Modelica Buildings library. The door class of the ModelicaBIM li-brarywraps DoorDiscretizedOpen class of the Buildings library. Detailedimplementations are explained in the following section.
4. ModelicaBIM library
The ModelicaBIM library consists of a ModelicaBIM Class packageand a ModelicaBIM Structure Example package. The Class package pro-vides wrapper classes of existing Modelica library — LBNL ModelicaBuildings library. Currently, the Structure Example package provides 5example Modelica models that represent Modelica BIM structures. Inthis section we explain each class in the Class package to describe theobject mapping process from BIM to BEM through the class wrappingprocess. Then, we explain the Structure Example package to describethe requiredmodeling process for creating a two-zonemodel withwin-dows and an interior door. The findings here will be a foundation to for-mulate a rule-based automated model translation from BIM to BEM.Dymola is used as a Modelica development and a simulation interfaceas shown in Fig. 4.
The left side pane is a package browser that shows a tree structure oftheModelicaBIMpackage. Themodel diagram in the right side showsTestCase 4 that has two rooms, an interior door, and twowindows. The yellowboxes are material, structure, and construction information of buildingcomponents, which are modeled with the classes in the ModelicaBIMclass package.
In the diagram, a series of icons are connected to each other by com-position lines. The icons represent physical components and the compo-sition lines represent connections among these components. Betweentwo room icons, a red line links two small red dots. The small red dotsare connectors and the line is the connect. The connector is a Modelicaclass that creates a physical flow between components. The connectclass is a communication interface between connectors of the compo-nents. A connect between two connectors is established as an equationinModelica using the following code: connect (connector1, connector2).
For instance, the door icon has four blue dots that are connected totwo room icons by four cyan lines. In the Modelica terminology, adoor component has four connectors. Two connects between the doorand each room are created to link two of four door connectors and tworoom connectors.
4.1. ModelicaBIM Class package
TheModelicaBIM Class package holds OOPM-based BIM classes thatrepresent building geometries, topologies, andmaterial properties. Cur-rently, basic classes for building components such as room, wall, roof,floor, window, and door are implemented for heat transfer throughbuilding envelopes and airflows between multi-zones.
While models in Revit and in Modelica are based on object-orientedmodeling concepts, they are independently created for respective imple-mentation considerations with specific object semantics and relation-ships. Such object-relational and semantic differences or mismatchescan cause conceptual and technical difficulties in the object-orientedprogramming process when creating a translator between object-basedapplications [36]. Compared to a translator between object-based appli-cation (e.g. Revit) and procedural-programming-based application (e.g.ASHRAE Toolkit for Building Load Calculations), the translator betweentwo object-based applications (Revit and Modelica-based thermalmodels) is a more natural selection that reduces the interoperabilityproblem during implementation [24]. The method to wrap predefinedclasses of the LBNL Modelica Buildings library results in new classes inour Class package so that (1) required parameters in Modelica can betransformed from Revit toModelica thermalmodels and (2) building to-pology in Revit can be translated into Modelica object relationship.
Fig. 4. Schematic model view of Test Case 4 in the ModelicaBIM library.
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Each class is created to contain (1) a set of parameters that can trans-fer parameters from Revit in Modelica, (2) instantiated classes of theLBNL library that hold equations, and (3) connects that link thewrapperclass and the wrapped class.
Energy model generation from BIM will translate building topolo-gies, geometries, and properties in Revit to Modelica. In creating new li-brary objects, we apply a common object relationship (Fig. 5) among
Fig. 5. Schematic object hierarchy of building
buildings, rooms, and building components that can facilitatetransformation of object hierarchy and classification from Revit toModelica.
A building consists of a series of spaces (Roomclass), each roomcon-tains roofs, floors, and walls (Construction class), each constructionconsists of a single or multiple material layer(s) (Structure class), andeach layer of the structure has material properties (Material class).
components in the ModelicaBIM library.
Table 2Test case models in the ModelicaBIM Structure Example package. The table also shows the workflows of designers (application users) and developers (the authors of the paper) in relation to the application (Revit2Modelica).
Test Case 1 Test Case 2 Test Case 3 Test Case 4 Test Case 5
Building Description
• One room model • Two room model • Two room model having windows • Two room model havingwindows and an interior door
• Three room, two story model havingwindows and an interior door
Create BIM objects (A1 in Fig. 2,by designers manually)
• Create four walls, a floor, and aroof object
• Create one room
• Create an interior wall based onTest Case 1
• Create two rooms
• Create windows on the exteriorwalls of Test Case 2
• Create a door on the interior wallof Test Case 3
• Create a room below the room ofTest Case 4
Generate Modelica instances in theModelicaBIM (A2, B1 in Fig. 2,by our system automatically)
• Generate instances of materials,layers, andconstructions
• Generate thermal zone instancesfor a single zone model
• Generate interior surfaceinstances
• Split two exterior surfacesintersected by the interior wall
• Split a floor and a roof into twopieces.
• Divide one zone into two zones.• Build an association of room-wall-room for heat transfercalculation
• Generate window instances• Assign new boundary condition oftwo walls having windows (fromexterior surfaces to exteriorsurfaces having windows).
• Generate a door instance• Generate an association of room-opening-room for air flow calcu-lation
• Assign an opening schedule
• Generate an instance of a lowerfloor thermal zone
• Generate a connection betweentwo zones
Develop Modelica classes in theModelicaBIM library (B2 in Fig. 2., byour researchers/developers)
• Develop classes of material, layer,and constructions of wall, roof, andfloor
• Develop a room class• Develop Modelica structure for thesingle zone model
• Develop material, layers, andconstruction classes of theinterior wall
• Develop Modelica structure forthe multi-zone heat transfermodel
• Develop material, layer, andconstruction classes of the window
• Develop Modelica structure foropaque constructions havingwindows
• Develop material, layer, and con-struction classes of the door
• Develop Modelica structure forthe multi-zone air flow model
• Develop material, layer, andconstruction classes of the lowerfloor thermal zone.
• Develop Modelica structure for themulti-zone heat transfer modelcontaining multiple stories.
Use Modelica classes and call solvers inthe LBNL Modelica Buildings library(B3 in Fig. 2., by our systemautomatically)
• Use classes for single zone thermalsimulation
• Call solvers for energy simulation
• Use classes for multi zonethermal simulation
• Call solvers for energysimulation
• Use classes for the window system• Call solvers for energy simulation
• Use classes for themulti-zone airflow model
• Call solvers for energysimulation
• Use classes for multi-zone thermalsimulation
• Call solvers for energy simulation
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Fig. 6. A Modelica code block for a thermal zone modeling.
23J.B. Kim et al. / Automation in Construction 50 (2015) 16–28
Constructions such as walls can host openings such as windows (Win-dow Structure classes) and doors (Door class). Material classes are in-stantiated as parameters in a Structure class, the Structure class isinstantiated as parameters in a Construction class, and they are instan-tiated as parameters in a Room class in the Modelica energy model. Thehierarchical components are described below.
Room: the Room class is a single-zone model to calculate heat ex-change through a building envelope such as convection, conduction, in-frared radiation, and solar radiation. Parameters such asfloor area, roomheight, and latitude are included in the Room class. To create the Roomclass, we wrap the MixedAir model in the LBNL Modelica Buildings li-brary, which is a model of a thermal zone with completely mixed air.The application and validation of this model have been conducted [29,30,37,38].
By wrapping existing classes, we assign new parameters and definenew object relationships among building materials, components, andtopologies in the Room class. To minimize an error-prone process inthe energy modeling, hard coded values in the existing LBNL ModelicaBuildings library are replaced with parameters in the Room classdeclaration.
The Room class can be used for a single thermal zone. When we in-vestigate thermal modeling for a single zone building, a room in BIM ismodeled as a single zone model in Modelica by using one Room object.By connecting multiple Room class objects, multi-zone thermal modelscan be created. One of our test cases demonstrates a multi-story build-ing by using two Room objects, which calculates conduction through aslab construction between two zones. In another test case, airflow be-tween two rooms is modeled by using a Door object and two Roomobjects.
Material: the Material, Structure, and Construction classes are creat-ed for material properties and geometry information of opaque con-structions. The Construction class instantiates the Structure class as aparameter and the Structure class instantiates theMaterial class as a pa-rameter as shown in Fig. 5.
The Material class represents thermal properties of a single materiallayer of opaque constructions. Parameters of this class include themate-rial thickness and physical properties such as thermal conductance, spe-cific heat capacity, and thermal resistance.Multi-layer constructions canbe created by using multiple Material objects in the Construction class.
Structure: the Structure class represents thermal properties ofopaque constructions having single or multiple material layers. TheStructure class uses the Material class objects as parameters. For amulti-layer construction, the corresponding number of Material objectscan be arrayed as parameters from the outside to the inside surface.Then the thermal resistance of all material layers is calculated in theStructure class. Other physical properties of outer and inner surfacessuch as absorptance and emittance are included as parameters in theStructure class.
Construction: the Construction class represents geometry informa-tion and physical properties of building components based on the Struc-ture class. Construction Classes for roof, floor, and wall have beenimplemented to create building geometry. Geometry information suchas area, tilt, and azimuth values are added as parameters and the valueswill be obtained from BIM.
Window: theWindow class represents a window systemwith glasspanels and window frames. Parameters for window geometry are in-cluded and the portion of a frame to the window system is included tostore the net frame area. A single pane glass is implemented currentlyand multi-layer glasses and window shading devices will be added toour library later.
Door: the Door class performs airflow simulation between twozones.We have implemented a closed door that calculates heat transferthrough the door and infiltration through the door assemblies. The doorclass consists of parameters of door geometry, a wrapped class of themulti-zone airflow class in the LBNL Modelica Buildings library, andconnect classes that present the building topology (room-door-room
association); the parameters are created to use door geometry informa-tion from BIM; the wrapped class supports a closed door, but an opera-ble and opened door can be simulated by using other classes directlyfrom the LBNL Modelica Buildings library.
4.2. ModelicaBIM Structure Example package
The ModelicaBIM Structure Example package holds OOPM-basedBIM models that present the energy modeling procedure by using theModelicaBIM Class package. Five test cases have been created in theStructure Example package to demonstrate how building design inBIM can be translated into the energy modeling in Modelica. Test Case1 presents a room model having six exterior surfaces. From this singlezonemodel, other models are sequentially created to demonstrate sim-ple operations in building design such as changing room geometry,adding an interior wall, and installing windows and doors as shown inTable 2.
Table 2 describes detailed modeling processes that can correspondto the Revit2Modelica workflow in Fig. 2. The associations betweenTable 2 and Fig. 2 are as follows.
• The second row, creating BIM objects, is related to A1 in Fig. 2. InRevit2Modelica, the development of extended BIM is manually doneby designers and some BIM objects are manually preprocessed.
• The third row, generating Modelica instances, is related to A2 and B1in Fig. 2. The ModelicaBIM Structure Example package defines re-quired rules of Modelica object instantiations and model creation sothat our system can translate BIM to ModelicaBIM models automati-cally.
• The fourth row, developing Modelica classes in the ModelicaBIM li-brary, is related to B2 in Fig. 2. The results of this task are theModelicaBIM class package.
• The fifth row, using Modelica classes and calling solvers of the LBNLModelica Buildings library, is related to B3 in Fig. 3. The ModelicaBIMStructure Example package defines how to use existingModelica clas-ses and call solvers so that our system can generate ModelicaBIMmodels and run the simulation automatically.
4.2.1. Creating a basic buildingmodelwith a single thermal zone (Test Case 1)Test Case 1 presents theModelicamodel structure and themodeling
procedure to create a single-zone model. To create a thermal zone,(i) building envelope information, (ii) boundary conditions of the
Fig. 7. A Modelica code block to connect two zones.
24 J.B. Kim et al. / Automation in Construction 50 (2015) 16–28
building envelope, and (iii) room geometry information need to be de-fined as shown in the Modelica code blocks in Fig. 6.
First, building envelope information is defined by using theMaterial,the Structure, and the Construction classes. In the code line 1, a roof ma-terial object is instantiated and values for physical properties are givento the parameters. Then a roof object is instantiated (line 3) and theroof material instance is used for a parameter (line 4).
Second, a boundary condition of the roof object is defined based onthe rules of the LBNLModelica Buildings library. Types of boundary con-ditionswe apply are opaque surfaces (datConExt), opaque surfaceswithwindows (datConExtWin), interior walls between two thermal zones(conBou, surBou), and interior partitions in a thermal zone (conPar).
Third, a roomobject is instantiated (line 7) and building envelope in-formation is given as parameters of the room object. Six surfaces of theroom are categorized as opaque surfaces (line 8) and their layer infor-mation (line 9), area (line 10), a tilt angle (line 11), and an azimuthangle (line 12) are provided.
4.2.2. Adding an interior wall (Test Case 2)Adding an interior wall in BIM splits a single zone into two zones in
the Modelica model. From an energy simulation perspective, (i) an ex-terior surface enclosing two zones needs to be divided into two surfacesas shown in Fig. 3 and (ii) heat transfer through the interior surface be-tween two zones needs to bemodeled. To split one zone into two zones,the second Room object is added. New wall, roof, and floor objects thatare intersected with the interior wall are also instantiated and thenadded to the two Room objects.
To build an interior surface in the Modelica model, materials, struc-ture, and construction information of the interiorwall are added. Then, aboundary condition of the interior wall is assigned as an interior surfacein the two Room objects (lines 3 and 6 in Fig. 7). Physical properties ofthe interior wall are added in the two Room objects. Lastly, the twoRoom objects are connected by using a Modelica connect that linkstwo rooms' connectors (line 11).
4.2.3. Adding a window (Test Case 3)When awindow is installed in an exterior wall in BIM, the boundary
condition of the exterior surface in the energy model needs to be up-dated to calculate heat flows through the window (e.g., solar radiationthrough glass panels, solar and infrared radiative heat exchange,
Fig. 8.Modelica connects for a
conduction through window frames, convection caused by glasspanes, infiltration through window assembles, and heat distributionfor infrared radiative heat gains).
In the Modelica model, (i) the boundary condition of the exteriorsurfacehavingwindows is updated fromanopaque surface (datConExt)to an opaque surface with windows (datConExtWin) within the Roominstance parameters, (ii) glass material objects are instantiated byusing a Glass class in the LBNLModelica Buildings library, and (iii) win-dow structure instances are created by using the Window class in theModelicaBIM Class package.
4.2.4. Adding an interior door (Test Case 4)An interior door in BIM can imply a room-door-room association
causing airflow between two zones through the door assembles andthe door openings in the energy model.
In the Modelica model, a door class in the LBNL Modelica Buildingslibrary is wrapped, parameters for the door dimensions are added, andModelica connects are created. Our door class implements a closeddoor that simulate airflow through a door assembles. The door objectcalculates bi-directional airflow between the door and a room, so twoModelica connects are required for linking a door and each of the tworooms; therefore four Modelica connects are created for two rooms(Fig. 8).
4.2.5. Adding stories (Test Case 5)In BIM, vertical stacking of multiple rooms in Test Case 5 and hori-
zontal expanding of multiple rooms in Test Case 2 are performedupon different approaches: creating a multi-story building in BIMneeds new floor plans at different levels with new building envelope el-ements. However, in BEM, heat transfer through an opaque surface be-tween two stories needs to be simulated, which is similar to the heattransfer between two rooms in Test Case 2. While two thermal zonesin the Test Case 2 are connected by an interior wall, two rooms in theTest Case 5 are connected by the slab between the two stories.
In theModelicamodel, the roof in the lower story and thefloor in theupper story are modeled as two opaque surfaces to perform conductionheat transfer. Then aModelica connect is defined to link the two opaquesurfaces.
door between two rooms.
Table 3Annual peak temperature of test cases.
Cases Room name Max temperature (°C)/Date Min temperature (°C)/Date
1 Room 33.88/Jul. 27th — 7 pm −20.90/Jan. 4th — 10 am2 East 33.00/Jul. 27th — 7 pm −19.46/Jan. 4th — 10 am
West 33.01/Jul. 27th — 7 pm −19.41/Jan. 4th — 10 am3 East 34.38/Jul. 27th — 5 pm −19.83/Jan. 4th — 7 am
West 33.39/Jul. 27th — 8 pm −19.68/Jan. 4th — 10 am4 East 34.22/Jul. 27th — 5 pm −19.18/Jan. 4th — 7 am
West 32.83/Jul. 27th — 7 pm −17.96/Jan. 4th — 10 am5 Upper 33.77/Jul. 27th — 5 pm −18.22/Jan. 4th — 7 am
Lower 32.02/Jul. 27th — 8 pm −18.21/Jan. 4th — 10 am
25J.B. Kim et al. / Automation in Construction 50 (2015) 16–28
5. Results and applications
To validate themappingmethod between BIM andModelicaBIM,weconducted simulation result comparisons between our library and theLBNL Modelica Buildings library. Five test cases are paired with twomodels: one is created by using the classes and model structures ofthe ModelicaBIM library and the other is modeled by using classes andmodel structures of the LBNL Modelica Buildings library.
We hypothesize that the two Modelica models of each test case cangenerate close or identical simulation results if theModelicaBIM classesand the ModelicaBIM Structure Example package are correctly createdin two conditions.
• First, our ModelicaBIM classes accurately wrap solver classes of theLBNL Modelica Buildings library by using the new parameters andthe new object hierarchy.
• Second, our ModelicaBIM structure Example package can be used tobuild the same architectural models as the LBNL Modelica Buildingslibrary.
Coherent model conditions across five test cases are as follows:
• The floor is not attached to the soil but above the ground level.• One room has a single thermal zone.• The building is located in the same location (Denver, Colorado, USA).• The building has no internal heat gains from occupancy and equip-ment.
• The door and windows are closed.• No HVAC systems are included.
All simulations have been performed using Dymola 2012 simulationprogram, LBNLModelica Buildings library version 1.3, a solver toleranceof 10−5, and a simulation interval of 3600 s for one year. First, building-level simulation results are compared. Then, building component-levelsimulation results are compared for the five test case models.
-20
-15
-10
-5
0
5
10
15
1 2 3 4
Tem
pera
ture
(C
)
Date
West room East room
West room (LBNL) East room (
°
Fig. 9. Indoor air temperature of Test C
5.1. Building-level energy simulation
In sum, the simulation results of all test casemodels created by usingthe ModelicaBIM library agree with results from the models createdwith the LBNL Modelica Buildings library: two models of each testcase show identical simulation results of annual indoor air temperatureand annual heat flow through conduction, convection, and radiation ofall five test cases. In Table 3, the highest temperature is obtained around5–8 pm on July 7th and the lowest temperature is found around 7–10 am on January 4th. The maximum temperature is 34.38 °C at theEast room of Test Case 3 on 5 pm, July 27th. The minimum temperatureis−20.90 °C in the room of Test Case 1 on 10 am January 4th. These re-sults also agree with those samples that we created using LBNLModelica Buildings library classes and their model structure.
Figs. 9 and 10 show an indoor air temperature of Test Case 4, a drybulb outdoor temperature, and a global horizontal radiation in thefirst week of February and August, respectively. Both our model andthe LBNL model have an east room with two windows and a westroom without windows. Each room of both models shows an identicalindoor temperature variation: two data series of each room are over-lapped on each other. While we present the Test Case 4, other testcases show the same indoor temperature change betweenModelicaBIMlibrary and the LBNL Modelica Buildings library.
The two rooms have similar patterns in the indoor temperaturecurves, but the west room curve shifts to right side of the east roomcurve. When the curves of global horizontal radiation and outdoor drybulb temperature are upward sloping, increasing of indoor temperaturestarts from east rooms because of their two windows. On the otherhand, dropping of indoor temperature also starts from east rooms dueto heat loss through their windows.
While these result comparisons are performed for validation of basiccases, more extensive validation of the simulation results was also con-ducted. We implemented a test case model using ANSI/ASHRAE Stan-dard 140–2007 that has been used for the building simulation toolvalidation [24]. Annual heating and cooling loads as well as their peakdays are compared to five simulation tools and the LBNLModelica Build-ings library. All the results are very close to LBNL's. In addition, the LBNLModelica Buildings library has been tested and validated [29] and it isconcluded that the simulation results of Buildings library are within therange specified in the ANSI/ASHRAE standards. Overall, theModelicaBIMlibrary generates similar results to the Buildings library.
5.2. Component-level energy simulation
Energy simulation using the ModelicaBIM library can calculatecomponent-level energy performances. We compared heat transferthrough individual building components of the five test cases, and the
01002003004005006007008009001000
5 6 7
Wh/
m2
Dry Bulb Temperature
LBNL) Global horizontal radiation
ase 4 in the first week of February.
01002003004005006007008009001000
0
5
10
15
20
25
30
35
40
1 2 3 4 5 6 7
Wh/
m2
Tem
pera
ture
(C
)
DateWest room East room Dry Bulb Temperature
West room (LBNL) East room (LBNL) Global horizontal radiation
°
Fig. 10. Indoor air temperature of Test Case 4 in the first week of August.
26 J.B. Kim et al. / Automation in Construction 50 (2015) 16–28
same simulation results are obtained. Figs. 11 and 12 show convectiveheat transfer values of Test Case 4 in a single day (February 7th). Be-cause of the same simulation results of each test case models in both li-braries, only the results of theModelicaBIM library are visible in figures.
The test case has two rooms and each room has six surfaces asshown in Table 2, so two plots present both the west and the eastroom temperatures. Convective heat transfer values of individual build-ing components are categorized as heat gain and heat loss and then theyare accumulated respectively.
In each bar chart, taller bars above zero imply higher convective heatgain and shorter bars below zero imply lower convective heat loss.Overall, the east room has higher heat gain and heat loss values thanthe west room. At the component level, one having a taller bar abovezero contributesmore to heat gain of a thermal zone than other compo-nents. For instance, the blue bars in Fig. 12 imply that the south wall ofthe east room contributes more to heat gain through convective heattransfer than other surfaces. The east room having windows drivesmore heat gain and loss than the west room due to the windows. Theheat flow of east room fluctuates more than thewest room; the highestand lowest values are shown in the east room; and the maximum valueis found around 11 a.m. in the east room and around 4 p.m. in the westroom.
Fig. 13 summarizes the annual convective heat transfer in accordancewith building components; 12 surfaces of two rooms in Test Case 4 arepresented. Both the south and the east wall having windows in theeast room contribute to heat gain and loss mostly, and both north wallsof the two rooms cause heat loss in general. The east room floor showsmore heat gain than the west room floor because of solar radiation ab-sorption from two windows. While we present only a couple of plots,more diverse component level simulation results can be generated and
Fig. 11. Convective heat transfer of the wes
associated with individual BIM objects, which can inform design-decision making to improve the overall building performance.
5.3. Discussion on simulation results
Our library and the LBNL's library yield identical simulation resultsfor the five test cases presented in this paper. However, different butvery close simulation results may occur when we change the order ofa room's components used as arguments in Modelica thermal calcula-tion functions.
As shown in the room object in Fig. 6, for example, we assign thebuilding component data as the room's parameters. In the five testcases, we input the roof data first, then floor, exterior walls, and interiorwalls— the same order as in LBNL'smodels.Whenwe switch the order ofthe components, different but very close simulation results are obtained.As an example, we switched the order of the roof and the floor and cal-culated the annual convective heat transfer differences between ourmodels and LBNL'smodels. The difference is defined as the area betweenthe twomodels' curves, representing the time series of heat transfer, di-vided by the area of the LBNL's model. For the roof of the west room inTest Case 4, the difference is 5.7%. For the floor of the same room, the dif-ference is 8.3%. In the building level in Test Case 4, the difference is 6.2%.Reducing model tolerance in Dymola will reduce these errors.
6. Conclusions
This paper presents the development and validation of theModelicaBIM library for BIM-based building energy simulation usingModelica in the scope of the building envelope. Based upon our method-ology, building topology, geometry, and materials in BIM can be
t room of Test Case 4 on February 7th.
Fig. 12.Convective heat transfer of the east roomof Test Case4 onFebruary 7th. (For interpretation of the references to color in thisfigure, the reader is referred to thewebversionof this article.)
27J.B. Kim et al. / Automation in Construction 50 (2015) 16–28
translated into Modelica building energy models. The library is used tocreateModelica building energymodels that can represent object seman-tics of BIM. Classes of theModelicaBIMpackage can store object attributesof BIM as their parameters and then use the information for energy sim-ulation. TheModelicaBIMstructures of our library can represent object se-mantics of BIM with same or similar object hierarchy among buildingcomponents.
Modelica energymodels using our classes andmodel structures gen-erate identical simulation results with models using the LBNL ModelicaBuildings library in result comparison (e.g. indoor temperatures andcomponent-level heat transfers).
Our methodology enables to produce component-level simulationresults that can indicate the performance of heat transfer of individualbuildng components. Coherent model structures and object hierarchybetween BIM and BEM allow direct feedback from energy analyses tobuilding design without complex result interpretation. Through the vi-sualization capabilities that BIM authoring tools provide, eachcomponent's performance can be visualized in the Revit models andtime-series simulation results can also be animated through custom ap-plications written with the Revit API [39].
As an intermediate library between BIM and Modelica, ourModelicaBIM library facilitates translation from BIM to ModelicaBEM; attributes of Revit models can be converted into energy model
Fig. 13. Annual convective heat transfer in the component level.
parameters in Modelica and then the parameters can be used in energysimulation. Using the LBNL Modelica Buildings library written inequation-based object-oriented modeling makes our implementationfaster than our previous approach on using Fortran-based ASHRAELoads Toolkit [24].
BIM andModelica have been developed not only by different partiesbut also with different perspectives.We emphasize the benefits and thepotentials of integrating them for BIM-based energy performance anal-ysis in the design stage. Our research will contribute to the advancedsystem development for sustainable building and community design.
For future work, new object classes and model structures will be in-vestigated for complex building geometry and the HVAC systems. A setof parameters within the ModelicaBIM Class package and a series ofmodeling rules in the ModelicaBIM Structure Example package will beutilized as new system interfaces between BIM and energy simulationthat perform automatic translation from BIM to BEM.
Acknowledgements
This material is based upon work supported by the National ScienceFoundation under Grant No. 0967446. Any opinions, findings, and con-clusions or recommendations expressed in thismaterial are those of theauthors and do not necessarily reflect the views of the National ScienceFoundation. We appreciate the valuable input from Sandeep Kota, JoseLuis Bermudez Alcocer, and the BIM-SIM Group, and the contributionfrom Mateo Aviles at Texas A&M University to the project.
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